The overall objective of this research proposal is to investigate intermediate hosts for Sarcocystis species with respect to host specificity and pathogenicity for normal and abnormal hosts. Previous work has shown that asexual stages of this parasite in intermediate hosts such as cattle, sheep and birds can be quite pathogenic, even fatal in heavy infections. The potential ability of Sarcocystis to infect abnormal hosts is important from a medical viewpoint because of the numerous infections of domestic animals to which humans may be accidentally exposed. Indirect evidence suggests that humans are sometimes accidentally infected with a non-human species and that the infection may be commonly overlooked. In this study, experimental infections with different Sarcocystis species will be utilized in small animals which can be monitored in the laboratory. Sites of asexual multiplication will be compared and correlated with pathology from a given infectious dose. In addition, the effect of chronic small infectious doses on the pathological response can be determined.